The basic structure of a GSM mobile-telephone system, for example, as described in “Digital Mobile Telephone Systems”, Dr.-Ing. Klaus David and Dr.-Ing. Thorsten Benkner, B. G. Teubner Stuttgart 1996, pages 326 to 341, is shown in simplified form in FIG. 5. In a mobile-telephone system of this kind structured according to the GSM standard, a mobile-telephone device, which is disposed, for example, in the motor vehicle 1 illustrated in FIG. 5, communicates with a base station 2.
In order to transmit information between the base station 2 and the mobile-telephone device, a downlink signal 3 is transmitted from the base station 2 to the mobile-telephone device, and an uplink signal 4 is transmitted back from the mobile-telephone device to the base station 2. To keep the downlink signal 3 and the uplink signal 4 separate from one another, both signals are transmitted with different carrier frequencies (e.g., FDD, Frequency Division Duplex).
The transmission of information in the downlink signal 3 and in the uplink signal 4 does not take place in a continuous manner, but in so-called bursts, wherein, eight such bursts of a respective downlink signal 3 or uplink signal 4 together form a frame. In FIG. 5, a downlink frame of this kind is indicated by the reference number 5, and the corresponding uplink frame is indicated by the reference number 6, wherein each burst is transmitted in a timeslot of a frame. The timeslots are numbered continuously from 0 to 7. The downlink frame 5 is transmitted with a first carrier frequency f1DL, and the uplink frame 6 is transmitted with a corresponding carrier frequency f1UL.
The information is transmitted only in individual bursts of the respective downlink signal 3 or uplink signal 4. For this purpose, one or more given timeslots 0 to 7 of the frame are assigned by the base station 2 to the mobile-telephone device of the motor vehicle 1. Each timeslot 0 to 7 of successive downlink frames 5 and uplink frames 6 forms a transmission channel for the exchange of information between the base station 2 and the mobile-telephone device of the motor vehicle 1. For the first carrier frequency f1DL and the corresponding carrier frequency f1UL of the uplink signal 4, there are therefore eight transmission channels, so that eight mobile-telephone devices can exchange information with the base station 2 independently from one another on this pair of carrier frequencies.
In addition to the first carrier frequency f1DL and the corresponding carrier frequency f1UL for the uplink signal 4, further carrier frequencies are provided for the downlink signal 3, and carrier frequencies corresponding to these are provided for the uplink signal 4. As a result of the TDMA structure with its eight timeslots 0 to 7 in one frame, there are therefore eight transmission channels for each of the 124 carrier-frequency pairs in the context of GSM 900 as illustrated in FIG. 5, wherein all transmission channels are independent of one another. The eight transmission channels for each carrier frequency pair, together with the 124 independent carrier frequency pairs, therefore provide a total of 992 transmission channels.
The use of one transmission channel simultaneously for several mobile-telephone devices to achieve an improved exploitation of the transmission capacity of a mobile telephone system of this kind is already known. Within a transmission channel, the mobile telephones are addressed by a base station, thereby specifying which of the several mobile-telephone devices receives data from the base station in which timeslots.
Corresponding timeslots of four successive frames of the downlink signal 3, or respectively the uplink signal 4, together form a transmission block of the respective transmission channel. For one respective transmission block, which is transmitted from the base station, it can be specified using an address signal ADR, to which of the mobile-telephone devices communicating with the base station in the same transmission channel the transmission block is transmitted from the base station.
Once again, in a simplified form, FIG. 6 illustrates a system of this kind. A total of eight mobile-telephone devices 7, which jointly use one transmission channel in order to communicate with the base station 2, are shown. This means that a given timeslot of the downlink frame 5 and the uplink frame 6 is used for the transmission of information between the mobile-telephone devices 7 and the base station 2. In order to transmit data in a transmission block from the base station 2 to a given mobile-telephone device 8, an address signal ADR, which respectively addresses a given mobile-telephone device 8, is transmitted in each transmission block of the downlink signal 9. By evaluating the address signals ADR, the mobile-telephone device 8 recognizes that the information contained in the transmission block is transmitted to this mobile-telephone device. The other mobile-telephone devices 7 do not recognize the address signal ADR as their own and reject the information of the transmission block. The base station 2 communicates to each mobile-telephone device 7, for example, at the time of establishing the connection, in which of the transmission blocks the mobile-telephone devices 7 actually evaluate an address signal ADR.
In this context, mobile telephones are all subscriber devices, which communicate with the base station 2. The correctness of the data of a transmission block received by the mobile-telephone device 8 is checked, for example, using a checksum. For each of the received transmission blocks, which were addressed to the mobile-telephone device 8, the base station 2 is notified on request, which transmission blocks were received and evaluated correctly. At the request of the base station 2, the mobile-telephone device 8 therefore transmits confirmation signals, for example, for each correctly-evaluated transmission block, a first marking “ack” (acknowledged), and for each incorrectly-evaluated transmission block, a second marking “nack” (not acknowledged). To achieve a correct communication of the complete information to the mobile-telephone device 8, each transmission block, for which the base station 2 has received, for example, a second confirmation signal “nack”, is re-transmitted.
In developing mobile-telephone devices and in testing devices in production, it is necessary to determine the number of transmission blocks received and evaluated incorrectly and to compare these with the number of transmission blocks transmitted and/or addressed as a whole to this mobile-telephone device. With a given level and given propagation conditions, a permitted maximum threshold of ten per cent (10%) is provided in the specification, for example, for an EGPRS, for an error rate (BLER, Block Error Rate) determined in this manner.